Nitrogen oxide removal control method

ABSTRACT

A plant with a gas turbine such as a combined cycle power plant of a gas turbine cycle and a steam turbine cycle is provided with a nitrogen oxide removal device for removing NO x  by injecting ammonia to an exhaust gas of the gas turbine. The device reduces NO x  concentration to a certain value or less before the exhaust gas is released to the air. An ammonia flow amount is rapidly controlled such that a mole ratio of ammonia to NO x  coincides with a set mole ratio value. The mole ratio is calculated from a predicted NO x  concentration at an inlet of the nitrogen oxide removal device predicted by calculating operation conditions of the gas turbine, an ammonia flow amount value, and an exhaust gas flow amount. The set mole ratio value is calculated from a deviation of a measured NO x  concentration value at an outlet of the nitrogen oxide removal device and a set NO x  concentration value, and an amount of water injected to a combustor. Finally, a controlled system is stabilized in a state where a measured NO x  concentration value coincides with the set NO x  concentration value at a high speed.

This application is a division of application Ser. No. 08/171,454, filedDec. 22, 1993 U.S. Pat. No. 5,449,495.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitrogen oxide removal controlapparatus and a method for reducing nitrogen oxide concentration in anexhaust gas by controlling an amount of ammonia which is injected to theexhaust gas of a gas turbine in a power generation plant.

2. Description of the Related Art

Recently, increasing demands for energy have caused a strong dependencyon fossil fuel. As the energy supply amount owing to fossil fuelincreases, CO₂ exhaust amount has increased. Thus, the crises of globalwarming has arisen and there is a worldwide move to put restrictions onthe CO₂ exhust amount.

From the above view point, a combined power generation plant is expectedto increase energy efficiency and allow reduction of CO₂. This system isconstructed by combination of a gas turbine and a steam turbine wheresteam is generated by using heat of exhaust gas of the gas turbine todrive the steam turbine.

As shown in FIG. 1, such a combined cycle power plant is provided with agas turbine cycle 1, an heat recovery steam generator 3 for generatingsteam by using exhaust gas 2 of the gas turbine cycle 1 as a heatsource, a steam turbine cycle 4 which uses this generated steam as adriving steam, and a chimney 5 for exhausting the heat-recoveredexhausting gas. In the gas turbine cycle 1, fuel 8 supplied from a fuelsystem is combusted with air 6 compressed by an air compressor 7 in acombustor 9. Combustion gas generated drives a turbine 10. A generator11 is connected to the turbine 10. After the exhaust gas 2 works,exhaust heat of the exhaust gas 2 is released to produce steam in theexhaust heat recovery steam generator 3 while the exhaust gas passesthrough an exhaust gas duct 12 to the chimney 5. The exhaust heatrecovery steam generator 3 has a superheater 13, an evaporator 14, anitrogen oxide removal device 15 and an economizer 16 along the upstreamside to the downstream side of the exhaust gas duct 12. Steam generatedin the superheater 13 is supplied to the steam turbine cycle 4 via asteam tube 17. In the steam turbine cycle 4, the steam coming out of aturbine 18 is condensed by a condenser 20. The condensate is introducedto the economizer 16 by a feed water tube 21, heated therein, evaporatedin the evaporator 14, and the steam is further heated in the superheater13. In the evaporator 14, while feed water is forcedly circulated ornaturally circulated by temperature difference, heat-absorption andevaporation are effected. In FIG. 1, although the turbine 18 isconnected to a generator 19, both the turbines 10 and 18 may beconnected to the same generator to construct single shaft type combinedcycle power plant.

In a plant with a gas turbine cycle 1 such as above combined cycle powerplant, firing temperature is preferably higher to increase the plantefficiency and relatively reduce CO₂. However, as firing temperatureincreases, nitrogen oxide (NO_(x)) emitted from a gas turbine cycle 1increases exponentially with the increasing temperature. Since thisnitrogen oxide (NO_(x)) is recognized as one contributing factor in airpollution, strict standards are applied to it's emission

Illustrative methods for reducing NO_(x) concentration are a methodwhere water or steam is injected to a combustor 9 to decrease firingtemperature, a method where fuel and air is mixed in advance and thenthe mixture is introduced to the combustor 9 to prevent a partial higherpart, and a method where a multistaged combustor capable of averagingcombustion temperature is used.

However, it is difficult with only these methods to achieve the NO_(x)emission standards. Thus, the nitrogen oxide removal device 15 isprovided in a flow path of exhaust gas to reduce NO_(x) emission. Thereis an ammonia injection/dry selective catalytic reduction decompositionmethod as one nitrogen oxide removal method applied to in this nitrogenoxide removal device. In the method, ammonia is injected to exhaust gasand the exhaust gas is passed through catalyst 22 placed on thedownstream side of the injection point so that nitrogen oxide is reducedand decomposited to non-toxic nitrogen component and water steam.Generally, this method has good reaction efficiency at 300° C. to 400°C. based on the temperature properties of the catalyst. Accordingly, thenitrogen oxide removal device 15 using this method is placed between theevaporator 14 and the economizer 16.

In this nitrogen oxide removal device 15, NO_(x) removal is controlledby adjusting an ammonia injection amount from an ammonia injectionsystem 23. U.S. Pat. No. 4,473,536 and U.S. Pat. No. 4,473,537 disclosecontrol system where a mole ratio of ammonia to NO_(x) is obtained byproportional integral (PI) control based on a deviation of a set NO_(x)value and a measured NO_(x) value, and this is multiplied by acalculated predicted NO_(x) value at an inlet of the nitrogen oxideremoval device to obtain an ammonia injection amount. However, in thiscontrol system, since a predicted NO_(x) value is multiplied, loop gainvaries dependently on the predicted NO_(x) value. If the predictedNO_(x) value becomes smaller, there is a tendency that loop gain is alsodecreased to degrade response.

Further, the other following control system has already been known. Inthis system, when load of a gas turbine and the like do not change, anammonia flow amount is controlled by feedback control loop which doesproportional integral operation based on a deviation of a set NO_(x)value and a measured NO_(x) value. When disturbance such as a loadchange of a gas turbine, which effects NO_(x) generation, is detected,an ammonia amount based on the detected disturbance amount is obtainedby feedforward control loop. This ammonia amount, which use as afeedforward control signal, is added to a feedback control signalobtained by proportional integral operation of a deviation of a setNO_(x) value and a measured NO_(x) value, thereby controlling an ammoniaflow amount. A delay time is about four minutes from a time when anammonia flow amount adjustment valve is opened or closed to a time whenthis opening or closing influences a measured NO_(x) value. On thecontrary, it takes about a second or less that exhaust gas of a gasturbine passes from the gas turbine to a chimney. Thus, when there isdisturbance such as a load change of the gas turbine, an ammonia flowamount cannot be controlled by the above-mentioned feedback controlloop. For this reason, according to this control system, whendisturbance such as a load change of the gas turbine is detected, arelay is activated, a contact of an output part of the above-mentionedfeedforward control loop is closed, and a contact between a proportionaloperating unit and an integral operating unit of the feedback controlloop is opened. As a result, input to an integral controller is stoppedto prevent unnecessary history from remaining in the integral operatingunit. However, a single shaft type combined cycle of a gas turbine andsteam turbine has problems that it is difficult to exactly detect loadchange of a gas turbine and a load change detecting relay, whichswitches opening/closing of each control loop contact, does not alwaysexactly operate.

SUMMARY OF THE INVENTION

An object of the invention is to provide a nitrogen oxide removalcontrol apparatus and method for removing nitrogen oxide by injectingammonia into an exhaust gas flow from a gas turbine, which overcomes theabove drawbacks of the related art.

Another object of the invention is to provide a nitrogen oxide removalcontrol apparatus and method capable of properly controlling a nitrogenoxide concentration in an exhaust gas from a gas turbine even in singleshaft type combined cycle power plants of a gas turbine cycle and asteam turbine cycle where it is difficult to correctly judge load changeof the gas turbine, separately.

Still another object of the invention is to provide a nitrogen oxideremoval control apparatus and method for an exhaust gas from a gasturbine with the excellent control performance independently of changein the gas turbine conditions, the apparatus and method using a measuredNO_(x) concentration signal, a representing state signal of a NO_(x)reducing means in a combustor of the gas turbine, an ammonia flow amountsignal, and a predicted NO_(x) concentration signal calculated from astate value of a gas turbine as control factors.

Further still another object of the invention is to provide a nitrogenoxide removal control apparatus and method for an exhaust gas from a gasturbine where feedback control based on the measured NO_(x)concentration signal and feedforward control based on a representingstate signal of a NO_(x) reducing means are fused by using the fuzzytheory.

The first aspect of the present invention provides a nitrogen oxideremoval control apparatus comprising a predicted mole ratio operatingsystem, a mole ratio setting system, and a mole ratio control system.The predicted mole ratio operating system provides a predicted moleratio of ammonia to NO_(x) in an exhaust gas on the basis of a predictedNO_(x) concentration value at an inlet of the nitrogen oxide removaldevice which value is calculated from state values of a gas turbine, avalue of the exhaust gas flow amount, and a value of the ammonia flowamount injected into this exhaust gas. The mole ratio setting systemprovides a set mole ratio value of ammonia to NO_(x) on the basis of adeviation of a measured NO_(x) concentration value at an outlet of thenitrogen oxide removal device and a set NO_(x) concentration value, anda representing state value of a NO_(x) reducing means in a combustor ofa gas turbine. The mole ratio control system manipulates an ammonia flowcontrol value on the basis of a deviation of an output from the moleratio setting system and an output from the predicted mole ratiooperating system.

According to this invention, an ammonia injection amount is controlledat a high speed such that a predicted mole ratio comes up to a set moleratio value, which control is performed by using a predicted NO_(x)concentration at an inlet of a nitrogen oxide removal device whoseresponse speed is high but the accuracy is slightly inferior. Further,the set mole ratio value is amended with a measured NO_(x) concentrationvalue at an outlet of a nitrogen oxide removal device whose responsespeed is slightly slow but the accuracy is superior. Thus, immediateresponse for disturbance such as a load change of a gas turbine ispossible, and finally a state where a measured NO_(x) concentrationvalue at an outlet of a nitrogen oxide removal device coincides with aset NO_(x) concentration value is established at a high speed andmaintained. Moreover, a mole ratio setting system can properlycorrespond to change with age of a controlled system. Further, the moleratio control system accurately responds to flow amount changes due todeterioration with age of an ammonia flow control valve because of thefeedback of an ammonia flow amount, so amending control of the moleratio control system may be a minimum. Thus, excellent controlproperties can always be maintained.

The second aspect of the present invention provides a nitrogen oxideremoval control apparatus comprising a predicted mole ratio operatingsystem, a mole ratio setting system, and a mole ratio control system.The predicted mole ratio operating system provides a predicted moleratio of ammonia to NO_(x) in an exhaust gas on the basis of a predictedNO_(x) concentration value at an inlet of a nitrogen oxide removaldevice which value is calculated from state values of the gas turbine,an exhaust gas flow amount value, and a flow amount of ammonia injectedinto this exhaust gas flow. The mole ratio setting system provides a setmole ratio value of ammonia to NO_(x) by the fuzzy inference on thebasis of a deviation of a measured NO_(x) concentration value at anoutlet of the nitrogen oxide device and a set NO_(x) concentrationvalue, and a representing state value change of a NO_(x) reducing meansin a combustor of a gas turbine. The mole ratio control systemmanipulate an ammonia flow control value on the basis of a deviation ofan output from the mole ratio setting system and an output from thepredicted mole ratio operating system.

According to this invention, in the mole ratio setting system, feedbackcontrol based on a deviation of a measured NO_(x) concentration value atan outlet of a nitrogen oxide removal device and a set NO_(x)concentration value, and feedforward control based on change in a stateamount of a NO_(x) reducing means are conducted by the fuzzy inference.As a result, according to change in a representing state value of aNO_(x) reducing means, transition between the feedback control and thefeedforward control can automatically and bumplessly be carried out.Namely, a flexible state where feedback control system can always serveas a backup for feedforward control system can be maintained. This moleratio setting system is combined with the predicted mole ratio operatingsystem based on a predicted NO_(x) concentration value which may containerrors but responds at a high speed by cascade system. As a result,speedy, correct and stable nitrogen oxide control can be performedagainst errors caused by a predicted NO_(x) concentration value, changein the properties with age and the like.

The third aspect of the invention provides a nitrogen oxide removalcontrol method, comprising the steps of operating a deviation of ameasured NO_(x) concentration value at an outlet of a nitrogen oxideremoval device, and a set NO_(x) concentration value; operating a setmole ratio value of ammonia to NO_(x) based on this deviation and arepresenting state value of a NO_(x) reducing means in a combustor of agas turbine; predicting a mole ratio of ammonia to NO_(x) in an exhaustgas based on an ammonia flow amount value, an exhaust gas flow amountvalue and a predicted NO_(x) concentration value at an inlet of anitrogen oxide removal device; and controlling an ammonia flow amount tobe injected into the exhaust gas based on a deviation of the predictedmole ratio and the set mole ratio value.

The fourth aspect of the invention provides a nitrogen oxide removalcontrol method, comprising the steps of operating a deviation of ameasured NO_(x) concentration value at an outlet of a nitrogen oxideremoval device, and a set NO_(x) concentration value; operating a changerate of a representing state value of a NO_(x) reducing means in acombustor of a gas turbine; calculating a set mole ratio value ofammonia to NO_(x) by the fuzzy inference based on the NO_(x)concentration deviation and the change rate of a state value; predictinga mole ratio of ammonia to NO_(x) in an exhaust gas based on an ammoniaflow amount value, an exhaust gas flow amount value and a predictedNO_(x) concentration value at an inlet of a nitrogen oxide removaldevice; and controlling an ammonia flow amount to be injected into theexhaust gas based on a deviation of the predicted mole ratio and the setmole ratio value.

According to the nitrogen oxide removal control method of the invention,even when nitrogen oxide properties change due to load change of a gasturbine, it is unnecessary to correctly detect the load change of a gasturbine. Flexible and safe nitrogen oxide removal control with highspeed and stable controlling properties can be realized.

The other objects, features and advantages will be apparent from thefollowing detail description referring to drawings, in which likereference numerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a structure of a combined powergeneration plant in which the inventor is used.

FIG. 2 is a block diagram showing one embodiment of a nitrogen oxideremoval apparatus according to the invention.

FIG. 3 is a view showing an example of membership functions in fuzzycontrol.

FIG. 4 is a list showing an embodiment of mole ratio setting rule ofFIG. 2.

FIG. 5 is a list showing an embodiment of mole ratio control rule of inFIG. 2.

FIG. 6 is a block diagram where an input signal is changed in theembodiment of FIG. 2.

FIG. 7 is a block diagram showing another embodiment of a nitrogen oxideremoval apparatus according to the invention.

FIG. 8 is a block diagram showing still another embodiment of a nitrogenoxide removal apparatus according to the invention.

FIG. 9 is a block diagram showing further still another embodiment of anitrogen oxide removal apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2, a nitrogen oxide removal control device 30a iscomposed of a mole ratio setting system 32, a predicted mole ratiooperating system 34, and a mole ratio control system 36. The device 30aprovides a valve opening degree control signal 38 to an actuator 42 ofan ammonia flow amount adjusting vale 40.

To the mole ratio setting system 32 are supplied a set NO_(x)concentration signal 44 related to a plant exhaust gas NO_(x) set pointwhich is determined in accordance with standard and the like, and ameasured NO_(x) concentration signal 46 related to a NO_(x)concentration in an exhaust gas of a nitrogen oxide removal deviceoutlet. An adder 48 produces a signal 50 of a NO_(x) deviation thereof.A water injection amount signal 52 related to an amount of waterinjected to the combustor 9 for lowering firing temperature is alsosupplied to the mole ratio setting system 32, and the signal 52 is timedifferentiated by a differentiator 54 to produce a water injectionamount change rate signal 56. A signal from the other proper means forreducing NO_(x) (NO_(x) reducing means) may be used instead of a waterinjection amount signal 52. Examples of such a signal include an amountof water steam injected instead of water, and ratio of a fuel flowamount of a main nozzle to total fuel flow amount when a two-stagedcombustor equipped with a pilot nozzle and the main nozzle is used.

The NO_(x) deviation signal 50 and the water injection amount changerate signal 56 are supplied to a fuzzy controller 58. The fuzzycontroller 58 has a fuzzy inference engine 60 and a mole ratio settingrule data base 62. The fuzzy inference engine 60 lets the input signals50 and 56 to be subjected to clustering in accordance with membershipfunctions as shown in FIG. 3 in dependence on a value E1 of the signal50 and a value DW of the signal 56, respectively. They are used as aninput to a conditional statements of the mole ratio setting rule database 62 and fire a corresponding rule of the mole ratio setting ruledata base 62 to perform fuzzy inference. This fuzzy inference provides avalue DM of a change rate of a set mole ratio of ammonia to NO_(x). Inthe mole ratio setting rule data base 62 which is applied to the fuzzycontroller 58, as shown in FIG. 4, if the value DW of the signal 56 islarge, e.g., during load change, feedforward control based on the signal56 is performed; and if the value DW of the signal 56 is small, bothfeedback control based on the NO_(x) deviation signal 50 and feedforwardcontrol based on the water injection amount change rate signal 56 areused. The two control systems automatically and bumplessly change toeach other.

A mole ratio change rate signal 64 supplied from the fuzzy controller 58is integrated by an integrator 66 to be changed to a mole ratio settingsignal 68.

To the predicted mole ratio operating system 34 are supplied an ammoniaflow amount signal 70 obtained by measurement, an exhaust gas flowamount signal 72 obtained by measurement or operation, and a predictedNO_(x) concentration signal 74 in an exhaust gas of a nitrogen oxideremoval device inlet calculated as high speeds from each kind of statevalue of a gas turbine 1. For example, this signal 74 can be calculatedby the method disclosed by U.S. Pat. No. 4,473,536 and U.S. Pat. No.4,473,537.

In a divider 76, the ammonia flow amount signal 70 is divided by theexhaust gas flow amount signal 72 to be changed to an ammoniaconcentration signal 70. This ammonia concentration signal 70 is dividedby the predicted NO_(x) concentration signal 74 in a divider 80 toproduce a mole ratio (signal 82) of an injected ammonia amount to apredicted NO_(x) amount of a nitrogen oxide removal device inlet. Thisratio is an output of the predicted mole ratio operating system 34.

In the mole ratio control system 36, an adder 84 provides a deviation ofthe set mole ratio signal 68 from the mole ratio setting system 32 andthe predicted mole ratio signal 82 from the predicted mole ratiooperating system 34. This mole ratio deviation signal 86 is supplied toa fuzzy controller 92 together with a mole ratio deviation change ratesignal 90 which is obtained by time differentiating the mole ratiodeviation signal 86 by a differentiator 88. The fuzzy controller 92 hasa fuzzy inference engine 94 and a mole ratio control rule 96. The fuzzyinference engine 94 lets the signals 86 and 90 to be subjected toclustering in accordance with membership functions as shown in FIG. 3 independence on a value E2 of the signal 86 and a value DE2 of the signal90, respectively. They are used as an input to conditional statements ofthe mole ratio control rule 96 as shown in FIG. 5 and fire acorresponding rule of the mole ratio control rule data base 96 toperform fuzzy inference. This fuzzy inference generates a value DU of avalve opening degree manipulating signal 98 of the ammonia flow controlvalve 40.

This valve opening degree manipulating signal 98 is integrated by anintegrator 100 to become the valve opening degree control signal 38which is provided to the actuator 42 of the ammonia flow control valve40.

In this embodiment, an ammonia injection amount is controlled at a highspeed by using a predicted NO_(x) concentration of a nitrogen oxideremoval device inlet whose response speed is high such that a predictedmole ratio comes up to a set mole ratio value. The set mole ratio valueis amended by a measured NO_(x) concentration value of a nitrogen oxideremoval device outlet whose accuracy is high. Finally, nitrogen oxideremoval control is stabilized at a high speed in a state where themeasured NO_(x) concentration value of a nitrogen oxide removal deviceoutlet coincides with a set NO_(x) concentration value.

As described above, according to the embodiments, the mole ratio settingsystem 32 is combined with the predicted mole ratio operating system 34by cascade system. Here, the system 34 is based on a predicted NO_(x)concentration signal 74 which may contain errors but responds at a highspeed, and the system 32 is based on a measured NO_(x) concentrationsignal 46 which has long delay time but correct. As a result, nitrogenoxide removal can correctly be controlled at high speed with theadvantages of both the system 32 and 34. In addition, nitrogen oxideremoval control can safely and stably be performed against errors causedby a predicted NO_(x) concentration signal 74 of the predicted moleratio operating 34, and change in the properties of a system to becontrolled with age, because the mole ratio setting system 32 can act asbackup. Further, this is most suitable for a single shaft type combinedcycle power plant of a gas turbine cycle and a steam turbine cyclebecause it is unnecessary to properly judge load change of the gasturbine. Moreover, in the mole ratio setting system 32, feedback controlbased on the NO_(x) deviation signal 50 and feedforward control based ona water injection amount change rate signal 56 are fused by using thefuzzy theory. Thus, there is no concept of conventional completeswitching between feedback control and feedforward control and aflexible state where feedback control can always act as backup, allowingflexible and safe nitrogen oxide removal control. Further, since controlgain is not changed by supplied control factors, stable nitrogen oxideremoval control with the excellent performance is always possible.

Referring now to FIG. 6, a nitrogen oxide removal controller 30b issupplied with a fuel flow amount ratio signal 102 of a main nozzle of atwo-staged combustor instead of the water injection amount signal 52which is supplied to the fuzzy controller 58 of the nitrogen oxideremoval controller 30a as shown in FIG. 2. The fuel flow amount ratiosignal 102 is changed to a change rate signal 106 of a fuel flow amountratio by a differentiator 104 like the water injection amount signal 52.The signal 106 is supplied to a fuzzy controller 108. The fuzzycontroller 108 fire a corresponding rule of a mole ratio setting ruledata base 112 which uses a value DR of the fuel flow amount ratio changerate signal 106 and a value E1 of a NO_(x) deviation signal 50 forconditional statement, and implement fuzzy inference by a fuzzy engine110 on the basis of the value E1 and DR. As a result, the controller 108produces a mole ratio change rate signal 64 (DM). Later steps are thesame as those of the nitrogen oxide removal controller 30a as shown inFIG. 2.

As described above, if the fuel flow amount ratio signal 102 of a mainnozzle in a two-staged combustor is used instead of the water injectionamount signal 52, the same actions and effects can be obtained.

Referring now to FIG. 7, as compared with the nitrogen oxide removalcontroller 30a as shown in FIG. 2, a nitrogen oxide removal controller30c further has a dead band 120 and a differentiator 122. The dead band120 allows a mole ratio deviation signal 86 to pass through, if theabsolute value of the mole ratio deviation signal 86 exceeds a certainvalue. The differentiator 122 time differentiates a NO_(x) deviationsignal 50. In addition, instead of the fuzzy controller 92 whichgenerates the valve opening degree manipulating signal 98 by fuzzyinference on the basis of the mole ratio deviation signal 86 and themole ratio deviation change rate signal 90, there is provided a fuzzycontroller 126 which generates a valve opening degree manipulatingsignal 98 by using a NO_(x) deviation signal 50 (El), a NO_(x) deviationchange rate signal 124 (DE1) obtained via the differentiator 122, a moleratio deviation signal 86 (E2) with the absolute value exceeding thecertain value via the dead band 120, and a mole ratio deviation changerate signal 90 (DE2) obtained by time differentiating this mole ratiodeviation signal 86 as input signals.

A deviation E1 of a set NO_(x) concentration signal 44 and a measuredNO_(x) concentration signal 46 is an input to the fuzzy controller 58for calculating a mole ratio change rate signal 64 as well as an inputto the fuzzy controller 126 for calculating a valve opening degreemanipulating signal 98. Further, the NO_(x) deviation change rate DE1(signal 124), which is obtained by time differentiating the NO_(x)deviation signal 50, is also supplied to the fuzzy controller 126.

The fuzzy controller 126 fires a corresponding rule of a mole ratiocontrol rule data base 130 by a fuzzy inference engine 128 to implementfuzzy inference a valve opening degree manipulating signal 98, independence on the NO_(x) deviation signal 50 (El), the NO_(x) deviationchange rate signal 126 (DE1), the mole ratio deviation signal 86 (E2)via the dead band 120 and the mole ratio deviation change rate signal 90(DE2) which are inputs of conditional statements of the rule 128.

If the absolute value of a mole ratio deviation signal 86, which isobtained by dividing a set mole ratio signal 68 by a predicted moleratio signal 82, is a certain value or less, the signal 86 is cut not tosupply to the fuzzy controller 126. At the same time, since adifferentiator 88 is on the output side of the dead band 120, a moleratio deviation change rate signal 90 is not also supplied to the fuzzycontroller 126.

Thus, if the absolute value of the mole ratio deviation signal 86 issmall, the fuzzy controller 126 conducts feedback control based on aNO_(x) deviation signal 50 (El) and a NO_(x) deviation change ratesignal 124 (DE1). This further increase the control performance.

Referring now to FIG. 8, a nitrogen oxide removal control device 30d isdifferent from the nitrogen oxide removal control device 30a as shown inFIG. 2 in the structure of a mole ratio control system 36. The device30d is provided with a proportional plus integral controller 132 whichgenerates a valve opening degree control signal 38 in dependence on amole ratio deviation signal 86.

In this structure, proportional plus integral control is performed suchthat a set mole ratio signal 68 coincides with a predicted mole ratiosignal 82. Here, the signal 68 is produced in dependence on a NO_(x)deviation signal 50 and a water injection amount signal 52 by a moleratio setting system and the signal 82 is produced from an ammonia flowamount signal 70, an exhaust gas flow amount signal 72 and a predictedNO_(x) concentration signal 74 of a nitrogen oxide removal device inlet.The same effects as those of the nitrogen oxide removal control device30a as shown in FIG. 2 can be obtained.

Referring now to FIG. 9, a nitrogen oxide removal control device 30e hasthe mole ratio setting system of which the structure is changed in thenitrogen oxide removal control device 30d as shown in FIG. 8. The moleratio setting system of the nitrogen oxide removal control device 30e iscomposed of a proportional plus integral controller 134, a feedforwardcontroller 136 and an adder 138. The proportional plus integralcontroller 134 calculates an ammonia/NO_(x) mole ratio based on a NO_(x)deviation signal 50. The feedforward controller 136 calculates anammonia/NO_(x) mole ratio based on a water injection signal 52. Theadder 138 adds an output of the proportional plus integral controller134 to an output of the feedforward controller 136 to supply a set moleratio signal 68.

The proportional plus integral controller 134 is composed of aproportional controller 140, an integral controller 142 and an adder144. The proportional controller 140 does proportional operation of aNO_(x) deviation signal 50. The integral controller 142 integrates anoutput of the proportional controller 140. The adder 144 adds an outputof the proportional controller 140 to an output of the integralcontroller 142. There are provided contacts 146a, 146b between theproportional controller 140 and the integral controller 142 and betweenthe feedforward controller 138 and the adder 136. The contacts 146a,146b are opened/closed inversely with each other. Generally the contact146a is closed and the contact 146b is opened. However, if change in awater injection amount is detected, a relay is activated so that thecontact 146a is opened and the contact 146b is closed.

Thus, if a water injection amount does not change, the contact 146b isopened. A set mole ratio signal 68 is then calculated on the basis of aNO_(x) deviation signal 50 such that a measured NO_(x) concentration (asignal 46) is the same as a set NO_(x) concentration (a signal 44).However, if a water injection amount which immediately influences NO_(x)concentration changes, the contact 146b is closed. Accordingly, a moleratio signal based on a water injection amount signal 52 is added to aset mole ratio signal 68, allowing nitrogen oxide removal control tofollow up the change in a water injection amount with little lag. Atthis time, since control based on a relatively time delayed NO_(x)deviation signal 50 is substantially ineffective, the contact 146a isopened. Input to the integral controller 142 is cut to preventunnecessary history from remaining in the integral controller 142.

This structure can also provide a nitrogen oxide removal controlapparatus with the excellent performance and stability.

Although the preferred embodiments as shown in the drawings aredescribed above, the present invention is not limited thereto. Variouschanges and modifications may be made in the invention without departingfrom the spirit and scope as set in the claims.

What is claimed is:
 1. A nitrogen oxide removal control method forcontrolling an amount of ammonia to be injected in a nitrogen oxideremoval device where ammonia is injected into an exhaust gas flow of agas turbine to remove NO_(x) by chemical reaction, the method comprisingthe steps of:obtaining a deviation of a measured NO_(x) concentrationvalue at an outlet of the nitrogen oxide removal device and a set NO_(x)concentration value; obtaining a set mole ratio value of ammonia toNO_(x) based on said deviation and a representing state value of NO_(x)reducing means in a combustor of the gas turbine; predicting a moleratio of ammonia to NO_(x) in the exhaust gas based on an ammonia flowamount value, an exhaust gas flow amount value and a predicted NO_(x)concentration value at an inlet of the nitrogen oxide removal devicecalculated from a state amount of the gas turbine; and controlling anamount of ammonia to be injected to the exhaust gas flow based on adeviation of the predicted mole ratio and the set mole ratio value.
 2. Anitrogen oxide removal control method for controlling an amount ofammonia to be injected in a nitrogen oxide removal device where ammoniais injected into an exhaust gas flow of a gas turbine to remove NO_(x)by chemical reaction, the method comprising the steps of:obtaining adeviation of a measured NO_(x) concentration value at an outlet of thenitrogen oxide removal device and a set NO_(x) concentration value;obtaining a representing state value change rate of NO_(x) reducingmeans in a combustor of the gas turbine; calculating a set mole ratiovalue of ammonia to NO_(x) by using fuzzy inference based on the NO_(x)concentration deviation and the state value change rate; predicting amole ratio of ammonia to NO_(x) in the exhaust gas based on an ammoniaflow amount value, an exhaust gas flow amount value and a predictedNO_(x) concentration at an inlet of the nitrogen oxide removal devicecalculated from a state amount of the gas turbine; and controlling anamount of ammonia to be injected to the exhaust gas flow based on adeviation of the predicted mole ratio and the set mole ratio value. 3.The method of claim 1, wherein the state amount of the NO_(x) reducingmeans is an amount of water or water steam injected to the combustor. 4.The method of claim 2, wherein the state amount of the NO_(x) reducingmeans is an amount of water or water steam injected to the combustor.